mostFrequentWord.swift

extension String {
	func mostFrequentWord() -> String? {
		return Dictionary(
			grouping: self
				.trimmingCharacters(in: CharacterSet.letters)
				.lowercased()
				.components(separatedBy: CharacterSet.whitespacesAndNewlines),
			by: { $0 }
		)
		.max(by:) { $0.value.count < $1.value.count }?.key // `key` is word, `value` is array of word, repeated `count` times
	}
}

print("Let's figure. out what the most comm$on wor!!!!d is in the sentence.".mostFrequentWord() as Any)